Method of making ceramic multilayer

ABSTRACT

The present invention provides a multilayer ceramic substrate and a method of producing the same which can prevent occurrence of cracks when laminating different ceramic substrates or due to subsequent heat shock, and which has excellent moisture resistance and insulation properties and high reliability. The ceramic multilayer substrate has two ceramic substrates of different thermal expansion coefficients laminated by at least two glass layers of different thermal expansion coefficients which are between the thermal expansion coefficients of the ceramic substrates so that the thermal expansion coefficient changes stepwise between the ceramic substrates. The differences in thermal expansion coefficient between the ceramic substrate and the glass layer which are adjacent to each other, and between the adjacent glass layers are preferably not more than 1×10 -6  /°C.

This is a division of application Ser. No. 08/576,440, filed Dec. 21,1995, now U.S. Pat. No. 5,705,260.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ceramic multilayer substrate whichcan constitute an electronic device or an electronic part.

2. Description of the Related Art

In recent years, ceramic multilayer substrates have frequently been usedas substrates for circuits which constitute an electronic device or anelectronic part. In order to realize high density and high integrationof electronic devices and electronic parts, some ceramic multilayersubstrates are in the form of a laminate of individual ceramicsubstrates having different characteristics, for example, combining alow-dielectric constant substrate with a high-dielectric constantsubstrate.

A typical example of such multilayer ceramic substrates comprises afirst substrate comprising a high-dielectric constant ceramic substratein which a plurality of capacitors are provided and connected to thesurface thereof by wiring and on which a thick film circuit has beenformed and a second substrate comprising a low-dielectric constantceramic substrate in which a plurality of capacitors and coils areprovided connected to the surface thereof by wiring with a thick filmcircuit formed on the surface thereof. The two substrates are laminatedby a resin or glass such that the opposing conductors are connected.Other electronic parts are mounted on the surface of the first or secondceramic substrates.

However, the conventional multilayer ceramic substrate comprising aplurality of ceramic substrates having different characteristics whichare integrated by laminating has the following problems:

(1) When the integrated ceramic substrates have thermal expansioncoefficients which greatly differ, the laminated portions of the ceramicsubstrates are stressed due to temperature drop or heat shock after theceramic substrates have been laminated by using melted glass, therebycausing cracks. In order to prevent such cracks, it is necessary thatthe thermal expansion coefficients of the ceramic substrates to belaminated coincide within each temperature range employed duringmanufacture and use. Attempts have made to add an additive to thesubstrates for controlling thermal expansion of the ceramic substrates.However, the addition of such an additive can cause deterioration of theother characteristics of the substrates.

(2) Although it is possible to prevent the occurrence of cracks by usinga resin having a low Young's modulus to relieve the stress generated inthe laminated portion, such a resin cannot be used where highreliability is required because the moisture resistance of the laminatedportion deteriorates.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide amultilayer ceramic substrate and a method of producing the same whichcan prevent the occurrence of cracks when laminating different ceramicsubstrates or due to subsequent heat shock, and which has excellentmoisture resistance and insulating characteristics and high reliability.

DESCRIPTION OF THE INVENTION

In order to achieve the above object, a ceramic multilayer substratecomprises units of two ceramic substrates which have different thermalexpansion coefficients and which are laminated by at least two glasslayers of different thermal expansion coefficients which are between thethermal expansion coefficients of the two ceramic substrates so that thethermal expansion coefficient changes stepwise between the two ceramicsubstrates.

In accordance with another embodiment of the present invention, aceramic multilayer substrate comprises a first ceramic substrate havinga passive element which is formed therein and connected to the surfaceby wiring, and a thick-film circuit formed on the surface; and a secondceramic substrate having a thermal expansion coefficient different fromthat of the first ceramic substrate, a passive element which is formedtherein and connected to the surface, and a thick-film circuit formed onthe surface; wherein the two ceramic substrates are laminated by atleast two glass layers of different thermal expansion coefficients whichare between the thermal expansion coefficients of the two ceramicsubstrates so that the thermal expansion coefficient changes stepwisebetween the ceramic substrates, the opposite conductors of the ceramicsubstrates are connected by conductors formed in the glass layers, andelectronic parts are mounted on the surface of the first or secondceramic substrate.

In accordance with a further embodiment of the present invention, amethod of producing a multilayer substrate comprises forming, betweenfirst and second ceramic substrates having different thermal expansioncoefficients, at least two glass layers having different thermalexpansion coefficients which are between the thermal expansioncoefficients of the first and second ceramic substrates so that thethermal expansion coefficient changes stepwise between the two ceramicsubstrates, and then laminating the first and second ceramic substratesby heating the glass layers.

In the multilayer ceramic substrate of the present invention, thedifference in thermal expansion coefficient between the ceramicsubstrate and glass layer, which are adjacent to each other, or betweenthe adjacent glass layers is preferably not more than 1×10⁻⁶ /°C.

The multilayer ceramic substrate of the present invention is produced bylaminating the ceramic substrates having different thermal expansioncoefficients by a plurality of glass layers so that the thermalexpansion coefficient changes stepwise. The glass layers decrease thestress caused by the difference between the thermal expansioncoefficients of the ceramic substrates as a result of a temperature dropor subsequent heat shock after the ceramic substrates are laminated byusing melted glass. The stepwise change may be substantially uniformsuch that the difference between any two adjacent materials is about thesame as the difference between any other two adjacent materials.

When the glass used is selected so that the difference in thermalexpansion coefficient between the ceramic substrate and glass layerwhich are adjacent to each other, or between the adjacent glass layers,is not more than 1×10⁻⁶ /°C., the stress generated is further decreased.

In addition, since the ceramic substrates are laminated by using glass,the resultant multilayer ceramic substrate has excellent moistureresistance and insulation properties, as compared to lamination using aresin.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view illustrating a multilayer ceramic substratein accordance with one embodiment of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

An embodiment of the present invention will be described below. Aceramic multilayer substrate in accordance with this embodimentcomprises a BaTiO₃ substrate having a high dielectric constant (ε=1000to 4000) and a thermal expansion coefficient of 10 to 12×10⁻⁶ /°C. (at25 to 500° C.), and a BaO--Nd₂ O₃ --TiO₂ substrate having a lowdielectric constant (ε=10 to 100) and a thermal expansion coefficient of8 to 9×10⁻⁶ /°C. (at 25 to 500° C.).

FIG. 1 is a sectional view of the ceramic multilayer substrate. In FIG.1, reference numeral 1 denotes a high-dielectric constant substrate,reference numeral 2 denotes a low-dielectric constant substrate, andreference numerals 3a and 3b each denote glass used for laminating thehigh-dielectric constant substrate 1 to the low-dielectric constantsubstrate 2. The high-dielectric constant substrate 1 has capacitors 4formed therein, and the electrodes of the capacitors 4 are led to thesurface by wiring. The low-dielectric constant substrate 2 has coils 5,a ground electrode 6 and internal wirings 7, which are formed thereinand which are led to the surface by wiring. A thick-film resistor 8 isformed on the surface of each of the high-dielectric constant substrate1 and the low-dielectric constant substrate 2, and surface mounted parts9 are mounted on the surface of the low-dielectric constant substrate 2.The circuits of the high-dielectric constant substrate 1 and thelow-dielectric constant substrate 2 are electrically connected by viaconductors 10, and external connecting end surface electrodes 11 areformed on the surface of the high-dielectric constant substrate 1.

The method of producing the ceramic multilayer substrate will bedescribed below.

The high-dielectric constant substrate 1 having the capacitors 4 formedtherein and the thick-film resistor 8 formed on the surface thereof isfirst formed as follows. An organic binder solution is added to a BaTiO₃ceramic raw material to form a slurry which is then used for forming aceramic green sheet (referred to as a "green sheet" hereinafter) by thedoctor blade method. Holes for via wiring are then formed in the greensheet. A capacitor electrode pattern and a wiring pattern including thevia conductors are formed on the green sheet by use of a conductorpaste. A predetermined number of the green sheets are stacked to form alaminate. The thus-formed laminate is then baked at 1300° C. to obtainthe high-dielectric constant substrate 1 having the capacitors 4 formedtherein and in electrical communication with the surface by the viaconductors (partly shown in the drawing). A circuit pattern is formed onthe surface of the high-dielectric constant substrate 1 by a conductorpaste, and the thick-film resistor 8 and the end surface electrodes 11are also formed on the surface.

On the other hand, the low-dielectric constant substrate 2 having thecoils 5, the ground electrode 6 and the internal wirings 7, which areformed therein, and the thick-film resistor 8, which is formed on thesurface thereof, is formed. An organic binder solution is added to aBaO--Nd₂ O₃ --TiO₂ ceramic raw material to form a slurry which is thenused for forming a ceramic green sheet by the doctor blade method. Holesfor via wiring are then formed in the green sheet, and a spiral coilpattern and a wiring pattern including the ground electrode and viawiring are formed by using a conductor paste. A predetermined number ofthe green sheets are stacked to form a laminate. The thus-formedlaminate is then burnt (baked) at 1250° C. to obtain the low-dielectricconstant substrate 2 having the coils 5, the ground electrode 6 and theinternal wirings 7, which are formed therein and communicating with thesurface by the via conductors (partly shown in the drawing). A circuitpattern is then formed on the surface of the low-dielectric constantsubstrate 2 by a conductor paste, and the thick-film resistor 8 is alsoformed on the surface.

Two glass pastes are formed by the following methods: An organic vehicleobtained by dissolving an acrylic resin or ethyl cellulose resin in anorganic solvent such as terpineol is added to a glass (referred to as"G1 glass" hereinafter) powder having a composition comprising 0.33 ZnO,0.50 B₂ O₃, 0.09 SiO₂, 0.04 LiO and 0.04 CaO (ratio by weight, softeningpoint 580° C.), followed by mixing the dispersion to obtain the G1 glasspaste. On the other hand, an organic vehicle obtained by dissolving anacrylic resin or ethyl cellulose resin in an organic solvent such asterpineol is added to a glass (referred to as "G2 glass" hereinafter)powder having a composition comprising 0.85 PbO, 0.11 B₂ O₃, 0.02 SiO₂and 0.02 Al₂ O₃ (ratio by weight, softening point 400° C.), followed bymixing the dispersion to obtain the G2 glass paste. Similarly, anorganic vehicle obtained by dissolving an acrylic resin or ethylcellulose resin in an organic solvent such as terpineol is added to anAg-based conductor powder, followed by mixing the dispersion to obtainvia conductor paste.

The high-dielectric constant substrate 1 and the low-dielectric constantsubstrate 2 are laminated by using G1 glass and G2 glass while beingelectrically connected by the via conductors 10. In other words, the G1glass paste is coated on a surface of the low-dielectric constantsubstrate 2, which is to be bonded to the high-dielectric constantsubstrate 1, by a screen printing method, and then dried. The G1 glasspaste is not coated on the via land portions for connecting conductorswhich is exposed on the surface of the substrate 2, but the viaconductor paste is coated on the via land portion and then dried. The G1glass paste layer and the via conductor paste layer are then burned(baked) on the low-dielectric constant substrate 2 at 610° C. to formthe glass layer 3a and the via conductors 10. The G2 glass paste iscoated on the glass layer 3a by the screen printing method, and thendried to form the glass layer 3b. The G2 glass paste is not coated onthe via conductors 10, but the via conductor paste is coated thereon anddried. A surface of the high-dielectric constant 1 to be bonded isplaced on the G2 glass paste of the low-dielectric constant substrate 2,and laminated thereto by melting the glass layer 3b by heat treatment at430° C.

The surface mounted parts 9 are then mounted on the surface of thelow-dielectric constant substrate 2 using solder or a conductor paste.

The glass layers 3a and 3b between the ceramic substrates 1 and 2prevent occurrence of cracks in the thus-obtained ceramic multilayersubstrate at the time of temperature drop after lamination of theceramic substrates 1 and 2, and improve the subsequent heat shockresistance. Since the ceramic substrates 1 and 2 are laminated togetherby the glass layers 3a and 3b, the composite exhibits excellent moistureresistance and insulating properties.

Examples of materials for the BaTiO₃ type high-dielectric constantsubstrate of this embodiment include a material comprising BaTiO₃ as amain component and Nb₂ O₅, Nd₂ O₃, Co₂ O₃ or SiO₂ added as a secondarycomponent thereto; and a material comprising BaTiO₃ as a main component,a bismuth compound such as Bi₂ O₃ --TiO₂, Bi₂ O₃ --SnO₂ or Bi₂ O₃--ZrO₂, and an oxide of a rare earth element. Examples of materials forthe low-dielectric constant substrate include the above-describedBaO--Nd₂ O₃ --TiO₂ type, MgTiO₃ --CaTiO₃ type and CaTiO₃ --La₂ O₃.2TiO₂--MgTiO₃ type and the like. The intended effect of the present inventioncan be obtained by laminating the different substrates selected from theabove examples.

Although this embodiment uses a combination of glass comprising 0.33ZnO, 0.50 B₂ O₃, 0.09 SiO₂, 0.04 LiO and 0.04 CaO (ratio by weight) andglass comprising 0.85 PbO, 0.11 B₂ O₃, 0.02 SiO₂ and 0.02 Al₂ O₃ (ratioby weight), the present invention is not limited to this combination.Namely, a plurality of glass materials having excellent insulationproperties and thermal expansion coefficients which are between thethermal expansion coefficients of the two ceramic substrates to belaminated can be selected from various glass materials such as leadborosilicate glass, bismuth borosilicate glass, zinc borosilicate glassand the like. The selected glass materials are disposed in layers sothat the thermal expansion coefficient changes stepwise, therebypreventing the occurrence of cracks due to a different between thermalexpansion coefficients. More than two layers may be employed.

When the types of glass used are selected, and the number of glasslayers is determined, so that the difference in thermal expansioncoefficient between the ceramic substrate and glass layer, which areadjacent to each other, or between the adjacent glass layers, is notmore than 1×10⁻⁶ /°C., the stress generated can be further decreased.

Although a thick glass layer used for laminating has the greatest effectin relieving stress, a thickness of not less than 5 μm is generallysufficient.

In order to securely remove the organic component in the glass pastelayers when the substrates are laminated by melting the glass byheating, it is preferable to previously decompose and remove the organiccomponent in the paste layers by heating beforehand, or to melt theglass layers by a heat treatment for a time sufficient to remove theorganic component.

As is obvious from the above description, the multilayer ceramicsubstrate of the present invention comprises ceramic substrates whichhave different thermal expansion coefficients and which are laminated bya plurality of glass layers so that the thermal expansion coefficientchanges stepwise between the ceramic substrates. The glass layers thuspermit the formation of a ceramic multilayer substrate which minimizesor prevents occurrence of cracks when there is a temperature drop orheat shock after lamination by melting glass.

When the types of glass used are further selected so that the differencein thermal expansion coefficient between the adjacent ceramic substrateand glass layer which are adjacent to each other or between the adjacentglass layers is not more than 1×10⁻⁶ /°C., the stress generated can befurther decreased, thereby further improving the crack resistance of theceramic multilayer substrate.

Since the stress generated due to the difference in thermal expansioncoefficient between the ceramic substrates is relieved, it is possibleto bond large substrates, which cannot be laminated by a conventionalmethod, and thus obtain a large integrated substrate and a parentsubstrate from which may be obtained many substrates.

In the present invention, a multilayer substrate having high reliabilitycan be obtained since the ceramic substrates are laminated by glasshaving good moisture resistance and insulation properties, and not by aresin adhesive, for relieving the difference between thermal expansioncoefficients.

What is claimed is:
 1. A method of producing a multilayer ceramicsubstrate comprising positioning, between first and second ceramicsubstrates having different thermal expansion coefficients, at least twoglass layers of different thermal expansion coefficients which betweenthe thermal expansion coefficients of the ceramic substrates such thatthe thermal expansion coefficient changes stepwise between the ceramicsubstrates, and then laminating the first and second ceramic substrateswith the glass layers therebetween by a heat treatment.
 2. A method forproducing a multilayer ceramic substrate according to claim 1 whereinthe first ceramic substrate has a passive element formed therein andconnected to a surface thereof by a conductor, and a thick-film circuitformed on a surface thereof; and the second ceramic substrate having athermal expansion coefficient different from that of the first ceramicsubstrate has a passive element formed therein and connected to asurface by a conductor, and a thick-film circuit formed on a surfacethereof; and connecting opposing conductors in the two substrates by viaconductors in the glass layers.
 3. A method of producing a multilayerceramic substrate according to claim 2, mounting an electronic part onthe surface of the first or second ceramic substrate.
 4. A method ofproducing a multilayer ceramic substrate according to claim 3, whereinthe glasses are selected such that the difference in thermal expansioncoefficient between a ceramic substrate and adjacent glass layer orbetween the adjacent glass layers is not more than 1×10⁻⁶ /°C.
 5. Amethod of producing a multilayer ceramic substrate according to claim 4,wherein the glasses are selected such that each of the stepwise changesin thermal coefficient are about equal.
 6. A method of producing amultilayer ceramic substrate according to claim 1, wherein the glassesare selected such that the difference in thermal expansion coefficientbetween a ceramic substrate and adjacent glass layer or between theadjacent glass layers is not more than 1×10⁻⁶ /°C.
 7. A method ofproducing a multilayer ceramic substrate according to claim 1, furthercomprising providing a first ceramic substrate having a first thermalexpansion coefficient, providing a second ceramic substrate having asecond thermal expansion coefficient, providing a first glass pastehaving a third thermal expansion coefficient which is between said firstand second thermal expansion coefficients, and providing a second glasspaste having a forth thermal expansion coefficient which is between saidfirst and second thermal expansion coefficient.
 8. A method of producinga multilayer ceramic substrate according to claim 7 wherein said glasspastes are heat treated before said positioning.
 9. A method ofproducing a multilayer ceramic substrate according to claim 8 whereinsaid first glass paste is positioned on said first ceramic substrate,said second glass paste is positioned on said first glass paste, andsaid second ceramic substrate is thereafter positioned on said glasspaste, and wherein the composite resulting from the positioning of atleast one of the first glass paste and the second glass paste is heatedto at least partly remove any organic components from said first orsecond paste or both before said second ceramic substrate is positioned.10. A method of producing a multilayer ceramic substrate according toclaim 1 wherein said glass layers comprise a plurality of glassmaterials selected from lead borosilicate glass, bismuth borosilicateglass and zinc borosilicate glass.
 11. A method of producing amultilayer ceramic substrate according to claim 1 wherein said glasslayers comprise a plurality of glass materials selected from leadborosilicate glass, bismuth borosilicate glass and zinc borosilicateglass.